“i need a device that i can use as a midi slave to the tempo of his stuff. i know the octatrack can do this.”
Yes, this is possible. However Ableton have numerous times been reported as a bit unstable in providing clock. I can´t vouch for this being the truth however (no attempts or experience with Ableton).

"can the octatrack store multiple rhythm guitar loops like this, to be played over the course of a set? are there presets that would allow me to go from one song to the next? i know the different tracks can hold different samples, but i get confused beyond that."
Yes, you can keep different guitar samples just like any other type of samples. There are no presets, as presets usually are known in i e rack processors. You do have Projects or Patterns. Each project could potentially almost be regarded as a “new” unit. One project could be setup working with samples only. Another working with the two PickUp machines only (with memory set to max for these).
Patterns on the other hand are close to what would be considered presets in a rack processors. Which means you´re able to recall them with normal program changes. However, each pattern are subjective to your project settings.

“midi foot controller to start and stop these pre-recorded loops. if there is a way to do this without one, please fill me in.”
You could setup Ableton to trigger all these loops by sending appropiate midi messages (note numbers, program changes and/or controller changes), they would always be related to Ableton though. May limit your freedom a bit and could mean potential problems if clock sync are being a bit finicky.

"i am wondering if i can use the octatrack as a live looper for my guitar stuff. i read that you must pre-determine the length of your loop before recording."
Depends a bit of where you´re coming from regarding experience of live loopers. It´s no Echoplex DPP, Looperlative or JamMan, even though it got some features that may remind you of them. You can reserve lenght of your recording time (so any flex samples won´t take up that memory), you can pre-determine lenght (so it fits with your OT sequencer). You can also use dynamic recorder lenght, but still that depends on what other stuff you´ve got loaded (flex samples). You could i e set an reserve lenght for R1 only (recorder 1), set that as dynamic in the memory settings and it would provide you with roughly 500sec (= 8 min). But then there´s nothing left for the other tracks. Remember this could be one of your projects, next project could be totally different in that regard. Still, the OT got a sequencer in it that it have to relate to (i e the grid). Live loopers doesn´t have that…

"easily accessible in a live setting, and practical enough to get used often."
That is totally up to WHAT you want to do. You may have to “construct” some workarounds regarding some features, that would be considered standard or “easy” with live loopers. But upside is that some features are unique in OT. You probably will have to adapt your mindset anyway…

“what aspects of the octatrack were designed with the guitarist in mind?”
Well, the PickUp machines weren´t necessarily designed with guitarist in mind (since they often need controllers anyway). But it was an adaptation of what´s already in there in other aspects. But many are using Flex recorders instead, since that gives you a few things that aren´t normally in an live looper (i e sequenced record/playback). All aspects got their pro´s and con´s…